The Diffusion Dialysis design that is described in detail herein, is used for the processing of acid solutions for the purpose of purifying the acid solutions, providing for the continuous recovery and reuse of the acid.
The principles and processes of Diffusion Dialysis dates back over several decades and operational systems have been in the field for at least 15-20 years. Diffusion Dialysis is a membrane separation process. Diffusion is a process in which a solute moves from an area of high concentration to an area of low concentration by virtue of the thermal energy of the system. Dialysis is a phenomenon in which a solute in solution permeates through a diaphragm. Solutes can be separated from each other by the differences in their diffusivity. The membrane acts as a diaphragm through which a solute at a high concentration will diffuse into a similar solvent containing lower concentrations of that solute. In practice, acid is the solute that diffuses through the membrane into water, the solvent. Diffusion Dialysis effects a migration, not an exchange, of anions and associated hydrogen ions, from an acid solution into water, based upon the concentration differences.
Anion exchange membranes are utilized in systems designed for the recovery of mineral acids from an acid/salt environment. Typically, the membranes are co-polymers of polystyrene and divinylbenzene and appear as thin sheets of wet, filmy plastic. Other polymers, co-polymers, and chemical compositions such as polyethylene may also be used. The membranes have been chemically prepared to possess anion exchange functionality. They also exhibit a strong affinity for acid absorption with no affinity for salt absorption.
The law of electroneutrality requires that ionic charge neutrality is maintained on both sides of the exchange membrane. To satisfy this law, either anions need to exchange through the membrane at an equal rate in both directions, or each anion that diffuses must be accompanied by an associated cation. Due to the extremely small physical size of the hydrogen ion, it is easily able to migrate through the perm-selective membrane in conjunction with an anion such as chloride, fluoride, nitrate, phosphate, or sulfate. The comparatively large metal ions are inhibited from associating with the diffusing anions; however, "leakage" of metal ions does occur.
The rate of metal "leakage" and the rate of acid recovery are functions of solution-membrane contact time. A balance is sought which maximizes the percent recovery of acid and minimizes the percent "leakage" of metal ions. Given the fixed membrane area in a dialysis stack, the recovery equilibrium is controlled by the solution flow rates.